I. Technical Field
The present invention relates to a hydrodynamic bearing type rotary device using a hydrodynamic bearing.
II. Description of the Related Art
In recent years, recording and reproducing apparatuses and the like using discs to be rotated have experienced an increase in memory capacity and an increase in transfer rate for data. Thus, bearings used for such recording and reproducing apparatuses are required to have high performance and high reliability to constantly rotate a disc load with high accuracy. Accordingly, hydrodynamic bearings suitable for high-speed rotation are used for such rotary devices.
The hydrodynamic bearing type rotary device has a lubricating oil between a shaft and a sleeve, and generates a pumping pressure by hydrodynamic grooves during rotation. Thus, the shaft rotates in a non-contact state with respect to the sleeve in the hydrodynamic bearing type rotary device. Accordingly, a stable high-speed rotation is possible because there is no mechanical friction between the shaft and the sleeve during constant rotation.
Hereinafter, an example of conventional hydrodynamic bearing type rotary devices will be described with reference to FIGS. 10 through 13.
As shown in FIGS. 10 through 13 a conventional hydrodynamic bearing type rotary device includes a sleeve 21, a shaft 22, and a rotor 23 fixed to the shaft 22.
The shaft 22 is inserted into a bearing hole 21A of the sleeve 21 so as to be rotatable. On at least one of an outer peripheral surface of the shaft 22 and an inner peripheral surface of the sleeve 21, radial hydrodynamic grooves 21A and 21B. Bearing gaps near the radial hydrodynamic grooves 21A and 21B are filled with at least oil 24.
Operations of the conventional hydrodynamic bearing type rotary device having the above-described structure are as follows.
In the conventional hydrodynamic bearing type rotary device as described above, when the shaft 22 rotates, the hydrodynamic grooves 21A and 21B gather the oil 24 filled in the bearing gap to generate a pumping pressure between the shaft 22 and the sleeve 21.
In this way, the shaft 22 can rotate in a non-contact state with respect to the sleeve 21. With a magnetic head or an optical head (not shown), data can be recorded/reproduced to/from a rotating disc 31.
However, the above conventional hydrodynamic bearing type rotary device has the following problems.
As shown in FIG. 10, a small gap is secured between the sleeve 21 attached to the base 25 and the shaft 22 and the oil 24 is filled therein. In such a conventional hydrodynamic bearing type rotary device, when the device is started and stopped many times under a high temperature (for example, 70° C.), a force such as self weight and the like is applied to the rotor 23 in a radial direction. This causes the surface pressure on a surface of the sleeve 21 or the shaft 22 to become significantly high. As a result, bearing wear may be generated and the bearing may seize.
Furthermore, as shown in FIG. 11, when the hydrodynamic bearing type rotary device is put into an axis horizontal position and the device is started and stopped many times, the self weight of the rotor 23 is applied to the surface of the sleeve 21 or the shaft 22 more strongly, and bearing wear is generated even earlier. As a result, as shown in FIG. 13 showing a chart of accumulative failure rate of actual products of the hydrodynamic bearing type rotary device with the letter E, the bearing has seized early in some cases.
Moreover, as shown in FIG. 12, even when a sleeve 26 has sufficient length, when the hydrodynamic bearing type rotary device is started and stopped, a force such as self weight and the like is applied to the rotor 23 in a radial direction. This causes a surface pressure on a surface of the sleeve 26 or a shaft 27 to become high. As a result, bearing wear may be generated and the bearing may seize with a failure rate as shown by the letter E in the chart of FIG. 13.
As can be seen from the above experiment results, in the radial hydrodynamic bearing of the conventional hydrodynamic bearing type rotary device, surface pressure increases when the device is started and stopped under a high temperature condition of about 70° C. This may result in bearing wear to be generated and the bearing may seize or may be broken. Regarding intermittence operation life of the hydrodynamic bearing type rotary devices, an experimental theory called PV value has been established with respect to sliding bearings of a non-floating type. It has been proven that as the numerical value of product of radial load and rotation rate increases, bearing wear may become significant and the life becomes shorter.
However, in bearings of a floating rotational type such as hydrodynamic bearing type rotary device, the relationship of the intermittence life has not been theoretically explained. It has been difficult to estimate the length of the intermittence life.